Process for improving the thermal shrinkage resistance of leather and product produced thereby

ABSTRACT

LEATHER HAVING DURABLE RESISTANCE TO SHRINKING ON EXPOSURE TO HIGH TEMPERATURES AS OCCASIONED BY FLASH FIRE IS PRODUCED BY PRETREATING THE LATHER BY IMMERSION IN AN AQUEOUS ALKALI METAL OR AMMONIUM SALT SOLUTION AND THEREAFTER IMPRENGATING THE PRETREATED LEATHER WITH A SALT OF A METAL SELECTED FROM THE GROUP CONSISTING OF THE TRANSITION METALS OF THE 4TH AND 5TH PERIODS OF THE PERIODIC TABLE AND MAGNESIUM. PREFERABLY A SOLUTION OF SODIUM 9 BICARBONATE, DISODIUMPHOSPHATE, DIAMMONIUM PHOSPHATE OR TETRA SODIUM PYROPHOSPHATE IS USED TO PRETREAT THE LEATHER AND NICKEL OR MAGNESIUM SALTS, ESPECIALLY THE SULFATES, ARE USED TO IMPREGNATE THE PRETREATED LEATHER.

United States Patent 3,824,124 PROCESS FOR IMPROVING THE THERMAL SHRINKAGE RESISTANCE 0F LEATHER AND PRODUCT PRODUCED THEREBY Alfred O. Minklei, Grand Island, and Sohn A. Peterson, Niagara Falls, N.Y., assignors to Hooker Chemical Corporation, Niagara Falls, N.Y. No Drawing. Filed May 4, 1972, Ser. No. 250,208 Int. Cl. C14c 9/00; C09k 3/28 US. Cl. 117-138 17 Claims ABSTRACT OF THE DISCLOSURE Leather having durable resistance to shrinkage on exposure to high temperatures as occasioned by flash fires is produced by pretreating the leather by immersion in an aqueous alkali metal or ammonium salt solution and thereafter impregnating the pretreated leather with a salt of a metal selected from the group consisting of the transition metals of the 4th and 5th periods of the Periodic Table and magnesium. Preferably a solution of sodium bicarbonate, disodiumphosphate, diammonium phosphate or tetra sodium pyrophosphate is used to pretreat the leather and nickel or magnesium salts, especially the sulfates, are used to impregnate the pretreated leather.

The invention herein described was made in the course of, or under, a contract or subcontract thereunder, (or grant) with the Department of the Air Force, United States Government.

This invention relates to methods for increasing the resistance of leather to shrinkage on exposure to heat. More particularly, the invention concerns the treatment, by impregnation, of leather with salts of certain metals whereby the leather has improved resistance to shrinkage on exposure to heat as by flash fires.

BACKGROUND OF THE INVENTION It is known that gloves, boots, jackets and other articles of apparel are often fabricated of leather and that such articles, especially gloves, tend to shrink excessively when exposed to high temperatures for relatively short periods, as in the case where they are exposed to a flash fire. The shrinkage may be so severe that the use of the hands or other portions of the body protected by glove or other article is seriously impaired. The result is that serious injuries have occurred when the wearer has been exposed to a flash fire.

Many processes for the fireproofing and shrink proofing of leather have been suggested by those skilled in the leather art. For example, US. Pat. 3,419,344 discloses a two step treatment of conventional mineral tanned leather whereby the leather is initially treated, retanned, with a phosphonium halide and then the retanned leather is subjected to a fat liquoring step with a phosphate ester. The initial 'retanning is said to impart resistance of the leather to afterglow while the second step aids in rendering the leather resistant to combustion. This procedure decreases high temperature shrinkage only slightly and does not significantly extend the use of the leather where high temperature dimensional stability is important. Other methods have not been wholly successful for the reasons that they impart undesirable eifects on the hand of the leather or the degree of improvement in resistance to high temperature thermal shrinkage is too small to substantially affect the leathers utility. Further, it is known (see Doklady Akad. Navk. S.S.S.R 82 4058 (1952) that conventional tanning processes do not significantly alfect the high temperature (dry collagen) dimensional stability, although the hydrothermal dimensional stability of the tanned leather may be improved.

3,824,124 Patented July 16, 1974 "ice In application Ser. No. 172,269 of George T. Miller, filed Aug. 16, 1971, now US. Pat. 3,775,162 and assigned to Hooker Chemical Corporation, it was disclosed that leather could be rendered resistant to shrinkage on exposure to high temperatures as occasioned by flash fires by impregnating the leather with a salt of a metal selected from the transition metals of the 4th and 5th periods of the Periodic Table and magnesium. The improvement in thermal shrink resistance however is not fast to washing, it having been ascertained that the metal salts can be leached out of the leather resulting in a reduction or gradual loss of the thermal shrink resistant character.

OBJECTS OF THE INVENTION It is, therefore, a principal object of this invention to provide leather having a resistance to shrinkage on exposure to high temperatures which is fast to Washing.

Another object is to devise a process for treating leather to improve its resistance to shrinkage on exposure to high temperatures which improvement is fast to washing.

Other objects will be obvious from the following description of the present invention.

BRIEF SUMMARY OF THE INVENTION These and other objects of the invention are accomplished by contacting leather preferably after it has been tanned and fat liquored, and which may or may not be finished, with an aqueous solution of an inorganic salt capable of forming insoluble metal salts such as sodium bicarbonate, disodium phosphate, diammonium phosphate or sodium pyrophosphate, and then treating the resulting leather with a solution of a salt of a metal selected from the group consisting of the transition metals of the 4th and 5th period of the Periodic Table or magnesium. Preferably this salt is nickel sulfate or magnesium sulfate.

The treatment baths are preferably warmed that is, from about 20 to about C. in each instance.

Sufiicient of the salt solutions should be used so as to impregnate the leather with an amount of salt which is at least 0.5% by Weight of the leather.

The treated leather thereafter may be dried in a moderate oven or by exposure to air.

In the description which follows glove leather will be used in the specific embodiments. It should be distinctly understood however, that the principles of the invention apply with equal force to other leathers, that is to say that the broad principles of the invention encompass the shrinkproofing of leathers irrespective of their intended use or classification. Thus, the invention applies to the treatment of skins and hides of all animals including cattle, horses, sheep, goats, pigs and the like. It is preferred to treat the leather in the tanned state, especially chrome tanned leather, and preferably after fat liquoring. Although the leather can be treated before or after finishing, it is preferred to treat it before finishing. The leather can be treated before or after fabrication into a useful article. Thus, for example, fabricated gloves can be treated by the present process, or the skin can be treated and then fabricated into a glove or other useful article.

DETAILED DESCRIPTION OF THE INVENTION In accordance with the present invention, leather, preferably that which has been subjected to the customary operations which follow the tanning step, is immersed in a dilute aqueous solution of an alkali metal or ammonium salt capable of forming insoluble metal compounds for a sufiicient time to permit thorough impregnation of the leather with the said salt solution, and thereafter the wet leather is immersed in a solution of a salt of a metal of the above defined group. The leather is worked in the solution until it is thoroughly impregnated with the treat- 3 I ing solution. The leather after removal from the solution may be rinsed and dried.

The leather is pretreated with an alkali metal or ammonium salt, capable of forming insoluble metal salts when contacted with a metal salt of the above defined group, for a period sufiicient to thoroughly impregnate the leather with the alkaline salt solution. Alkali metal or ammonium salts, such as sodium bicarbonate, disodium phosphate, diammonium phosphate, sodium pyrophosphate are suitable examples of such salts. Sodium bicarbonate because of its relative effectiveness, general availability and low cost is preferred as the pretreatment salt for this invention. The time of immersion in this solution will vary according to the size of the leather article being treated, in general periods of from 5 to minutes or longer being adequate. The concentration of the salt solution for this pretreatment step is not critical. Concentrations of from about 5 to about 15% are effective, although more concentrated solutions may be used if desired. The temperature used in this pretreatment step is not critical, temperatures about ambient being suitable, although it is preferred to pretreat the leather in solutions at temperatures of from about to about 85 centigrade.

Preferably the leather to be treated, is a metal salt tanned leather and especially that which has been fat liquored. At this stage in manufacture, the leather is referred to as in the crust. The leather may also be treated after subsequent finishing steps such as drying, lacquering, etc.

Following the pretreatment with an alkali metal or ammonium salt solution the leather is treated with a metal salt of the type defined above. This metal salt is preferably an aqueous solution, although organic solvent solutions which do not dissolve out the fat liquor can be used. Solven s such as alcohol, acetone, methyl ethyl ketone, and the like can be used. The solvent should be one, or a mixture thereof, in which the metal salt is soluble to the extent of at least one percent by weight.

Suflicient of the solution of the metal salt should be used so that the impregnated leather after treatment will contain at least 0.5 percent of the metal salt by weight. By metal salt in this instance is meant the metal compound formed by reaction of the pretreatment alkali metal or ammonium salt with the metal salt of the above defined group.

The temperature of the treatment bath may be varied over a wide range, e.g., from about 5 degrees centigrade to about 95 degrees centigrade. Preferably the treatment bath is at a temperature within the range of about 20 to about 85 C. At lower temperatures, the rate of impregnation may be impractically slow whereas at high temperatures, e.g., approaching the boiling temperature of the bath, the leather may undergo shrinkage before the shrinkproofing character of the invention has been imparted to the leather.

Conveniently, the treatment solution can be an aqueous solution of the metal salt at a concentration within the range of about five percent by weight and about forty percent by weight. Saturated solutions can be used although such are not necessary and hence are not to be preferred.

The metal salts generally used contain one or more of the transition elements of the 4th or 5th period of the Periodic Table or the alkaline earth metal, magnesium. Thus salts of nickel, cobalt, chromium, magnesium, manganese, are typical of the metal salts which can be used. Copper and zinc salts have also been found to act similarly.

The metal salts found effective in this invention can contain a wide variety of anions. Suitable anions may be inorganic or organic and include the chlorides, bromides, sulfates, phosphates, carbonates, nitrates, oxyhalides, formates, acetates, propionates, oxalates, carbamates, tartrates, stearates, oleates, palmitates and the like, as well as mixtures and hydrates thereof. Further, the metal salt can be complexed with complexing agents such as am- .4 1 monia, quinoline, pyridine, and various amines and the like. Additionally, double salts can be used such as copper potassium tartrate, nickel potassium sulfate, magnesium sodium sulfate and the like. Nickel and magnesium are the preferred metals for usein this invention and, in a most preferred embodiment, are used in the form of their sulfates and/or various hydrates.

It is believed that the metal salt reacts with the alkali metal or ammonium salt in the leather matrix precipitating an insoluble salt which is resistantto leaching. Thus, when sodium bicarbonate is used basic or normal metal carbonates may be precipitated in the leather matrix in situ such as, NiCO -2Ni(OI-I) -H O or MgCo -3H O. Similarly, insoluble precipitates may be formed when alkali metal or ammonium phosphates are used with nickel of magnesium sulfates or other suitable metal salts.

It has been found that more extensive and better retained precipitates were formed when the leather is first treated with the alkali metal salt solution, followed by soaking in the metal sulfate solution rather than treated first With the metal sulfate solution and then immersed in the alkali metal salt solution. It has also been found to be preferable especially when larger pieces, as for example whole skins, are being treated that the treated leather be allowed to stand for an extended period, for example, up to about 16 hours or more, before rinsing rather than immediately removing the soluble salts by rinsing. It is believed that a slow diffusion of the reactants and substantially complete conversion into the insoluble salts are favored by this holding time, and the durability of the treatment, i.e., resistance of the treated leather to rinsing with water, is thereby increased. It is further found in the case of magnesium salts that the formation deposition rate within the leather can be enhanced by including some sodium bicarbonate in the magnesium sulfate treating solution, as an alternate to the overnight standing.

The treatment vessels used for both the pretreatment and impregnation steps can be any vessel in which the leather can be placed in intimate contact with the solution. Open or closed vats preferably provided with means to gently agitate the leather during the treatments should be used. Rotating vessels, e.g., tanning drums, are convenient for these treatments. The treated leather can be washed with cold to warm water or solvent after impregnation and before drying, if desired.

The time of the impregnation step can be varied considerably. The period of treatment in this step should be adequate to permit reasonably thorough penetration of the metal salt solution. As will be evident to those skilled in this art, the porosity of leathers vary over a wide range depending upon the type and pretreatment thereof. Under normal conditions, a chrome tanned cow hide leather can be impregnated in a warm, e.g., 50 to 60 centrigrade, solution for a period of from about 10 minutes to one hour. Longer periods of time can be used, but such are usually without sufiicient additional beneficial effect as to warrant the additional effort.

As indicated above, the impregnated leather should preferably be permitted to stand for an extended period before rinsing. This period may vary from about 5 to 48 or more hours. Preferably and conveniently, about 16 hours, that is overnight standing, is generally sufficient.

Conveniently, for relatively small pieces of leather, the impregnated leather may be placed in a polyethylene bag and closed, and the material permitted to stand at ambient temperature for the required period of time. Alternately, the leather pieces may be loosely stacked and placed in a container, the container closed and the pieces held therein for the desired period. This latter procedure is especially convenient for the treatment of larger leather pieces, e.g., whole skins. Other means for accomplishing this procedure will be readily apparent to those skilled in this art.

The following examples will illustrate the present invention. Unless otherwise indicated, all parts and percentages are by weight and temperatures are given in degrees'centrigrade. j 1 EX MPLE I Samplesymeasuring 1 /2 x 1 /2 Iin., of chrome tanned cabretta leather-were immersed for ten minutes in a ten percent aqueous solution of sodium bicarbonate at 25 degrees. The samples.v were removed from the bath, drained and then soaked for twenty minutes in a 20% aqueous solution of magnesium sulfate at 50 degrees and at' a"ipI-I of -5.5.- The impregnated leather samples were their 'rinsed-in--water' at ambient temperature for 7 minutes, and driedin air. The treated leather square was suspendedg by.hooks 'under light tension at the center of a 3 inch diameter liole in a test table. A shutter beneath the-sample wasopened to allow'a 3.0 second exposure to the flameof-theFisher burner having a 1 /2 inch diam- .eterto'pi fiame temperature ab'out'1-300 degrees. For comparison an untreated leather sample was exposed in the same inan'neri "Ihkfarea shrinkagefor the treated samples was an average" of'-'about16.5-%' -The untreated sample shrunk'about 60%.

Samples measuring" 1 /2 it'l /2 in. of chrome tanned cabretta leather wereirnmersed in a 10% aqueous s0lution of sodium bicarbonate for ten minutes at 25 degrees. The pretreated samples weregiven a fast rinse in cold Water and then soaked for 5.0 minutes at 50 degrees in a 20.6% aqueous solution oft-nickel sulfate at a pH of 5.5. The impregnated leather" samples were rinsed in cold water for about seven minutes and then air dried. The dried samples-.were exposed to the flame of a Fisher burner as in Example I and found to shrink an average of about 12 /2 percent 4 I EXAMPLE III Samples measuring 1 /2 x 1 /2 in. of cabretta leather were immersed in a 10% aqueous solution of diammonium phosphate at 25 degrees for minutes. The samples were then soaked in a 20.6% solution of nickel sulfate for 50 minutes at 50 degrees and at a pH of 5.5. The impregnated leather samples were rinsed in cold water for seven minutes and air dried. When exposed to the flame of the Fisher burner as in Example I above, the samples had an average area shrinkage of 19%.

For comparison similar leather samples were first soaked for 50 minutes in a 20.6% solution of nickel sulfate at 50 degrees and at a pH of 5.5. The impregnated leather samples were quickly rinsed in cold water and then immediately soaked in a 10% aqueous solution of diammonium phosphate for twenty minutes at degrees after which they were rinsed for seven minutes in cold Water and then air dried. On being flamed as described above, the samples shrunk an average of 25.3%.

EXAMPLE IV Several squares measuring 1 /2 in. x 1 /2 in. of cabretta leather were soaked for ten minutes in a 10% aqueous solution of disodium phosphate at 23 degrees, and then for 20 minutes in a 20% aqueous solution of magnesium sulfate at 50 degrees and a pH of 3.5. The impregnated samples were rinsed in cold water for seven minutes and then air dried. Upon being exposed to Fisher burner as described in Example I above, the samples showed an average area shrinkage of 24% A similar series of cabretta leather samples were soaked for ten minutes in a 10% aqueous solution of sodium phosphate at 23 degrees and then for 50 minutes at a 20.6% aqueous solution of nickel sulfate at 40 degrees and a pH of 4.0. The impregnated samples were then rinsed for ten minutes in cold Water and then air dried. On being flamed as described 6 EXAMPLE 'v Cabretta leather samples measuring 1 /2 x 1 /2 inches were immersed for twenty minutes at 50 degrees in a 20% solution of magnesium sulfate at a pH of 5.5. The impregnated samples were given a fifteen second rinse in cold water and then soaked for ten minutes in a 10% aqueous solution of disodium phosphate for ten minutes at 25 degrees. The leather samples were rinsed for 15 seconds in cold water and then immersed for 5 minutes in a 5% aqueous solution of ammonium hydroxide at 25 degrees. The treated samples were rinsed for seven minutes in cold Water and then air dried. On being exposed to the flame of a Fisher burner as described in Example I above, the samples shrunk an average of 20 area percent.

EXAMPLE VI Cabretta leather samples were soaked for twenty minutes at 25 degrees in a 20% aqueous solution of tetrasodium pyrophosphate and then for 20 minutes in a 20% aqueous solution of magnesium sulfate at 50 degrees and a pH of 6. The impregnated samples were rinsed for sixty minutes in cold water and then air dried. The average area shrinkage of the samples flame as described in Example I above 24%. This experiment was repeated with the exception that the leather samples after soaking in tetrasodium pyrophosphate were immersed in a 20.6 solution of nickel sulfate for 50 minutes at 50 degrees at a pH of 5.5. After being rinsed for 60 minutes in cold water and air drying, the average area shrink-age of the flame samples was 30%.

EXAMPLE VII A full size cabretta leather skin was immersed for 30 minutes in a 9% aqueous sodium bicarbonate. solution at 25 degrees. The leather was gently agitated in the bicarbonate solution during the soaking period. Thereafter, the leather was immersed in a 20% aqueous solution of magnesium sulfate maintained at 50 degrees for two hours and at a pH of 6. During the impregnation step, the leather was agitated intermittently in the impregnating solution. The treated skin was then stored overnight in a crumpled condition in a polyethylene bag, after which it was washed for 1 hour in 10 gallons of water at 15 degrees. The treated skin was then dried in air. Samples 4 in. x 4 in. in dimension were flamed as described in Example I above, and showed an average area shrinkage of 21%.

EXAMPLE VIII Samples measuring 4 in. x 4 .in. of chrome tanned cabretta leather was soaked in a 10% aqueous sodium bicarbonate solution for 30 minutes at 25 degrees. The samples were then immersed in a solution containing 20% magnesium sulfate and 5% sodium bicarbonate (pH of 6.9) at 50 degrees and held therein for one hour. Thereafter, the samples were rinsed in cold water for 1 hour and air dried. On being exposed to the flame of a Fisher burner the samples showed an average area shrinkage of 15%.

EXAMPLE IX Chrome tanned cabretta leather samples measuring 4 x 4 inches were soaked in a 10% aqueous sodium bicarbonate solution for 30 minutes at 25 degrees. The samples were then immersed in a 25% aqueous nickel sulfate solution (pH 5.5) and held therein for 2 hours at 25 degrees. The impregnated samples were removed from the nickel sulfate solution, drained and stored in a covered container for about 16 hours at ambient temperature. The samples were rinsed in water for 2 hours and then air dried. On being exposed to the flame of a Fisher burner as in Example I above, the average area shrinkage was 21 percent.

The invention has been described and illustrated in the several examples given above, but such are not intended to limit the invention. As will be apparent to those skilled in this art, various changes and modifications can be made in the process details set out in the above without departing from the scope and spirit of this invention.

What is claimed is:

1.. A process for improving the thermal shrinkage resistance character of leather which comprises the steps of treating said leather with an aqueous salt solution of a member of the group consisting of disodium phosphate, diammonium phosphate and sodium pyrophosphate and with an aqueous solution of a salt of nickel or magnesium for a time sufficient to impregnate said leather with at least 0.5% by weight of the insoluble salt formed by the reaction of the nickel or magnesium salt with the phosphate or pyrophosphate salt.

2. The process of Claim 1 wherein said first recited salt solution is a diamrnonium phosphate solution.

3. The process of Claim 1 wherein said first recited salt solution is a disodium phosphate solution.

4. The process of Claim 1 wherein the salt used in the second recited salt solution is a nickel salt.

5. The process of Claim 4 wherein the nickel salt is nickel sulfate.

6. The process of Claim 1 wherein the salt used in the second recited salt solution is a magnesium salt.

7. The process of Claim 6 wherein the magnesium salt is magnesium sulfate.

8. The process of Claim 1 wherein the impregnated leather, prior to rinsing and drying, is permitted to stand for at least about 16 hours in an enclosed area.

9. The process of Claim 1 wherein the leather is first treated with the nickel or magnesium salt solution and thereafter with said other recited aqueous salt solutions.

10. The process of Claim 9 wherein the leather is first impregnated with a nickel salt solution.

1.1. The process of Claim 10 wherein the nickel salt is nickel sulfate.

12. The process of Claim 9 wherein the leather is first impregnated with a magnesium salt solution. H

13. The process of Claim 12 wherein the magnesium salt is magnesium sulfate. v

14. The process of Claim 1 wherein the leather is first treated with said first recited aqueous salt solution and thereafter with the nickel or magnesium salt solution.

15. Leather obtained 'by the process of Claim 1.

16. Leather obtained by'the process of Claim 9.

17. Leather obtained'by the process of Claim 14.

References Cited' 1 UNITED STATES PATENTS 3,038,818 6/1962 Findlay 117 142 X 3,102,772 9/1963 Robinson 117 142 X 3,463,665 8/1969 Quinn 117-138 2,188,746 l/l940 Bersworth 8 94.26 X

2,999,768 9/1961 Boresch 117 142 X FOREIGN PATENTS 180,758 6/1922 Great Britain 117 142 OTHER REFERENCES Chakravorty, H. P. et al., Journal of the Society of Leather Trades Chemists, vol. XLI, 1958, pp. 2-10.

LEON D. ROSDOL, Primary Examiner H. WOLMAN, Assistant Examiner US. Cl. X.R. 

